Optical fiber cables are well known in the communication industry as cables that include one or more optical fibers for optically transmitting communication signals.
Among other constructions, one of the popular arrangements for optical fibers cables is a bundling of six to twelve individual optical fibers within a tube (also referred to as a buffer tube) in a loose arrangement, allowing for some movement of the optical fibers within the tube. This is referred to as a “loose tube” arrangement. Moreover, to form the optical fiber cable, one or more tubes may be bundled within an outer cable jacket for additional protection from the environment and also to provide an increased number of fibers within a particular cross section, useful for commercial installations.
However, there are several competing concerns that affect the design and production of such optical fiber cables. The first of these concerns is the optimum amount of fibers per tube. In typical installations larger optical fiber cables have multiple tubes therein. The greater the number of fibers per tube, the greater the overall communication capacity for the optical fiber cable. However, more fibers per tube may result in difficulty accessing individual fibers within a tube (e.g. for connection to optical equipment). Furthermore, more fibers add weight to the cable as well as geometrical constraints, both of which add costs in the form of materials and production difficulties.
A related second drawback to existing optical fiber cables of this design is the attenuation in fiber signals that occur when the optical fiber cable is bent. Attenuation occurs when individual fibers within an optical fiber cable are bent resulting in the optical signals partially or totally exiting the fiber at bending regions. Increases in the number of fibers within each of the tubes in an optical fiber cable and their consequent geometric configuration, however restricts the possible movements of the fibers during bending, causing awkward and strained bending resulting in attenuation.
FIG. 1 shows an exemplary prior art arrangement of an optical fiber cable having seven fiber tubes within a jacket. FIG. 2 shows a hypothetical bend of the fiber cable depicted in FIG. 1. The centrally located tubes (b) can conform to the center of the bent cable, but tubes along axes (a) and (c) are either stretched or compressed, resulting in signal attenuation. Thus, the more fibers placed in fiber optic cable the more attenuation in the fiber signal, particularly with fibers closer to the inside wall of the cable jacket.
Given the constraints associated with attenuation from bending, combined with the desire to meet customer communication throughput needs by providing sufficient fibers per cable, prior art optical fiber cables are designed to include a limited number of fibers per tube (typically between 6 and 12 fibers per tube). However, even with this range of fibers per tube, the attenuation at bend radiuses that may result in significant signal attenuation.
To address this problem, prior art designs include either strength members or binding ribbons to resist bending (or to prevent over-bending as some bending is required). Other designs have added fillers such as petroleum jelly or other gels, in either the tubes or around the tubes in the jacket. U.S. Pat. No. 4,230,395 discusses an example of such gel filled tubes. Yet another method of preventing attenuation in the fibers in these cables is to strand the fibers in a helical or S-Z arrangement so that no one fiber is consistently disposed along the far side of a bend axis.
All of these solutions are less than desirable. The addition of strength members adds additional construction components, adding cost in both materials and cable construction complexity. Furthermore, the strength members add additional weight to the final product. The addition of gel fillers also adds cost in both materials and extrusion complexity, adds weight, as well as the additional drawback of a fire fuel, which contributes to such gel filled cables failing the necessary fire safety standards for certain indoor uses.
Stranding, adds significant cost to the production of a cable in that the twisting of the fibers requires that more fiber per foot of cable is necessary to span a given distance relative to a straight or non-stranded fiber cable. Also, in the stranded arrangement, fibers acquire an inherent wavy quality that includes a certain amount of bending, which can result in failure of the cladding to contain the light signal through reflection, resulting in undesired attenuation.
In addition to the above identified drawbacks associated with fiber optic cable design, such as 12 fiber cables, another attribute that is difficult to address is Skew and PMD (Polarization Modal Dispersion).
Skew refers to the detrimental time difference caused by optical signals traveling over different length fibers within the same cable over the same cable distance. This is caused by fibers having different fiber lengths relative to one another within the same length of cable.
For example, digital signals are often broken into multiple paths (ie different fibers within a cable) with the expectation that they will be re-assembled into the original set in the correct order at the other end of the cable. This requires the ability to compensate on the receiving end for any variance in the arrival time between the paths.
Receiving circuits can manage this variance, but as the rate of digital signal throughput is increased to the 40-100 gigabit range it becomes more important that the various path (fiber) lengths, physical composition, and subjected stress be as equal as possible to one another so as not to exceed the delay disparity management capabilities and “correction budget” of the electronics on the receiving end of the cable.
Currently there is no agreement in the art on the best method to measure skew. Some methods focus more on the fiber length differential, while others include all the various factors that may contribute to time delay. These methods include the Phase Shift method and Pulse Time-of-Flight method but there are other existing methods.
Polarization Mode Dispersion (PMD) is related to the feature of a communication signal whereby the signal is a modulated beam of optical light with the x and y axis arrival time being affected by variances in the glass refractive indexes in the x and y axes. This variance in the x and y axes refractive indices is called bi-refringence. PMD is related to the differential group delay (DGD) caused by this birefringence in optical fibers.
In order to avoid Skew and PMD, it has been found to ideally have all of the fibers within a cable to be of equal or nearly equal length and also to have the lowest retained stress in both the relaxed (straight) and bent/flexed states.
In the prior art, as shown in FIG. 9, one manner to address this issue is with ribbon cables, proponents of which note that the fusing of the fibers in the ribbon arrangement assures equal or nearly equal length over the course of the cable. Although the ribbon arrangement is good at keeping all of the fibers within the cable parallel and of equal length when the ribbon is flat during installation in real-world environments, the twisting, bending and coiling imparts significant stress on the fibers.
For example, when the skew of fiber ribbons are measured in a relaxed parallel state, skew results may be in the range of 0.5 to 3.0 ps/m (picoseconds/meter) based on length differential tolerances in the range of 0.01%-0.04% between fibers within the cable. However, under coiling and bending stresses, these Skew results increase by 5-15 ps/m for the reasons outlined above.
In another prior art arrangement, as shown in FIG. 10, another manner to address Skew and PMD in twelve fiber-loose tube arranged cables is to periodically bind the fibers to one another with either a binder or glue so that the overall length of the fibers remain relatively equal along the length of the cable.
However, even with the periodic binding, residual length differences caused by the manufacturing process (e.g. payoff tensions of 5-15% in the fibers during cable production) may render the fiber length differentials in the range of 0.08% to 0.16%. Such length differentials typically result in Skew measurements in the range of 4-8 ps/m.
Although initially, the Skew results are for periodic binding more than the ribbon design, unlike ribbons, the Skew results for the bound loose tube arrangement only increase in the range of an additional 3-5 ps/m during bending and installation because the fibers are free to move (relax) after installation.